1. Field of the Invention
The invention relates to a method for the collective production of microlenses at the end of a set of optical fibres, of the ribbon of fibres type.
The present invention applies to optical and optoelectronic modules amongst other things for optical telecommunications. It applies more particularly to the production of microlenses on optical fibres in order to improve the coupling between optical and optoelectronic components. These microlenses are particularly well adapted to collective coupling with active components in arrays, such as lasers, semiconductor amplifiers, VCSELs or photodetectors for example.
2. Discussion of the Related Art
In the literature a large number of articles are found having methods for the individual manufacture of microlenses at the end of fibres which improve the coupling between active components and monomode fibres. The history of these microlenses is presented in the collection of publications xe2x80x9cMicrolenses Coupling Light to Optical Fibersxe2x80x9d, Huey-Daw Wu, Frank S. Barnes, 1991, pp. 149-213: xe2x80x9cMicrolenses Coupling Light to Optical Fibersxe2x80x9d IEEE Lasers and Electro-optics Society 1991 [1].
On the other hand, very few articles are found concerning collective coupling lenses.
The most recent articles report on combinations of lengths of fibres of different natures and the fashioning of a lens at the end of fibres, but always in order to produce individual microlenses.
In fact, individual coupling lenses are known. Reference can be made to the article by K. Shiraishi et al. (University of Utsunomiya, Japan) xe2x80x9cA fiber with a long working distance for integrated coupling between laser diodes and single-mode fibersxe2x80x9d, Journal of Lightwave Technology, Vol. 13 No 8, pp. 1736-1744, August 1995 [2], which presents a lens whose working distance is 160 xcexcm for laser-fibre coupling losses of 4.2 dB and lateral and angular axial positioning tolerances respectively of 35 xcexcm, 2.6 xcexcm and 0.8xc2x0 for an additional loss of 1 dB. The results were obtained for a laser emitting at a wavelength of 1.49 xcexcm with a mean total half-height divergence of 20.5xc2x0 (that, is to say 34xc2x0 at 1/e2). This is a length of fibre 1 without a core and with a hemispherical end, welded to a monomode fibre 1 whose core has been locally enlarged by heat treatment, as shown by FIG. 1.
In a more recent article, Shiraishi and Hiraguri xe2x80x9cA lensed fiber with cascaded Gi-fiber configuration for efficient coupling between LDs to single-mode fibersxe2x80x9d ECOC ""98, 20-24 September, Madrid Spain, pp. 355-356 [5], propose a new lens consisting of two lengths of monomode fibres, of different natures, whose focusing parameters are different, welded together and to a monomode fibre by electric arc. A hemispherical profile is conferred on the end multimode fibre by means of an electric arc welder. Losses of 2 dB are obtained in front of a laser diode emitting at 1.3 xcexcm, whose total divergence in far field at half-height of the maximum is 24.90xc3x9719.50 (that is to say 42.2xc2x0xc3x9733.1xc2x0 at 1/e2). The working distance is 50 xcexcm.
If the publications concerning individual fibre laser coupling lenses are numerous, those dealing with collective lenses intended for multichannel optical modules are more rare.
A method is known which consists in interposing an array of microlenses (not fixed to the fibre ribbons). By way of example, the coupling lens shown in FIG. 2 of G. Nakagawa and al. (Fujitsu Laboratories, Japan) xe2x80x9cHighly efficient coupling between LD array and optical fiber array using Si microlens arrayxe2x80x9d IEEE Photonics Technology Letters, Vol. 5, No 9, pp. 1056-1058, September 1993 [4], makes it possible to obtain 4.8xc2x10.3 dB by dynamic coupling between the array 4 of four lasers with a total half-height divergence of 30xc2x0 (that is to say 44xc2x0 at 1/e2) and 4 monomode fibres 21, 2n by means of a matrix of silicon lenses. This type of coupling complicates the assembly steps, since it adds an additional component to be positioned very precisely.
In 1996, J. Le Bris xe2x80x9cHigh performance semiconductor array module using tilted ribbon lensed fibre and dynamical alignmentxe2x80x9d ECOC ""96 Oslo THc.2.3, p. 4.93, from the company Alcatel (AAR, France) proposes a lensing method on a fibre ribbon which consists of chemically etching a ribbon of monomode fibres and reworking the end of each fibre of the ribbon by electric arc. With this method 3.6 dB of loss is obtained in front of an array of semiconductor amplifiers with ribbons misaligned by 20xc3x9725xc2x0 of total half-height divergence (that is to say 34xc3x9742.5xc2x0 at 1/e2). The wavelength is 1.55 xcexcm.
The recommended solutions for the xe2x80x9clensingxe2x80x9d of the fibres (the fitting of lenses at the end of fibres) which make it possible to obtain good coupling levels are not collective methods in the case of references [1] to [3].
In addition, the outside diameter of the 125 xcexcm fibre is not maintained all along the microlens, which poses a problem for the hybridisation on a silicon platform in precise positioning Vs and for precision ferrule fitting.
For the collective methods known at the present time, the coupling losses are still too high. In addition, the use of discrete lenses described in reference [4] requires several successive alignments, which increases the number of assembly steps compared with microlenses attached at the end of the fibre. The method described in reference [5] also imposes a very short working distance of less than 15 xcexcm in addition to the fact that it is complex.
The purpose of the present invention is to improve the coupling between an array of active elements and a set of aligned fibres of the fibre ribbon type.
To this end, the invention relates to a method for the collective production of microlenses at the end of a set of aligned optical fibres, principally characterised in that it comprises a step of heating the end of all the fibres by means of an electric arc in order to form the microlenses, the plane in which the ends of the fibres are situated being distant from the line of hottest points of the electric arc in order to round their end evenly.
The method according to the invention also has the advantage of being collective and therefore compatible with mass production, and with a very high performance.
According to another characteristic of the invention, the distance between the optical fibre ends and the line of hottest points is between 850 micrometres and 950 micrometres.
Advantageously, the set of optical fibres consists of a ribbon.
According to a preferred embodiment of the invention, the ribbon comprises monomode fibres whose terminations comprise a length of silica welded to a length of fibre with an index gradient, the microlenses being produced at the end of the lengths of fibres with an index gradient.